The present invention relates to a method for producing an absorbent structure for use in an absorbent article, such as a sanitary napkin, incontinence guard, diaper and like articles.
Many different types of absorbent articles of this kind are known to the art. The absorbent body of such articles is produced conventionally by dry-defibering cellulose pulp contained, for instance, in reels, bales or sheets, and converting the cellulose pulp in a fluff state to a pulp mat, sometimes while mixing so-called superabsorbents in the pulp mat, these superabsorbents being polymers which are capable of absorbing many times their own weight in liquid or body fluid.
EP 0,122,042 describes a method of manufacturing an absorbent structure which includes superabsorbent material. The cellulose pulp is first defibered to form cellulose fibres, so-called fluff pulp, whereafter a mixture of these fibres and water-insoluble hydrogel particles are air-laid to form a web from which the absorbent structure is formed and subsequently compressed to a density of 0.15-1.0 g/cm.sup.3.
The pulp body is compressed with the intention of reducing its bulk and also with the intention of enhancing its liquid wicking properties. With the majority of sanitary products a thin article is desired so that the article can be worn as discretely as possible.
Superabsorbent material is normally used in the form of granules or small particles. It may be found difficult to bind these small particles satisfactorily in the absorbent structure, which is one problem associated with the use of superabsorbent material. The method described, for instance, in EP 0,122,042 involves the risk of superabsorbent material being spread in and around the manufacturing plant, and also involves the risk of particles of superabsorbent material escaping from the finished pulp core when packaging, transporting or using the core. When manufacturing products which include superabsorbent material, the aforesaid problems become greater with thinner products.
Superabsorbent particles often have sharp edges and are therefore at times able to perforate the liquid-impermeable sheets of absorbent articles, unless preventative measures are taken. It is also important to obtain intimate contact between fibres and superabsorbent material, in order to achieve effective transference of liquid between the two materials.
Furthermore, the absorbent body may include other ingredients that are intended, for instance, to improve its ability to take-up or disperse liquid, or to increase its coherency and its ability to resist deformation while in use.
A very large part of the production plant used by the manufacturers of the aforesaid sanitary articles is comprised of defibrating equipment, pneumatic conveyor systems and mat-forming equipment. This production plant is often followed by equipment for compressing the finished pulp mat or the finished sanitary product.
It is known from U.S. Pat. No. 3,017,304 that a surface sheet containing paper bonds and having a higher density and greater strength than the original cellulose pulp mat can be obtained, by spraying water onto a cellulose pulp mat and then compressing the mat. Such a layer is sometimes called a "Burgeni layer". It is also known from U.S. Pat. No. 4,186,165 to compress a cellulose pulp mat that contains superabsorbent material with the aid of heated rolls and without adding water, and therewith obtain in the cellulose pulp mat an inner layer which has a higher density and greater strength than in the original pulp mat. The particles of superabsorbent material are bound to the structure in this layer, and the layer can also be obtained at a desired level in the structure by varying the temperature difference between the rolls. However, when practicing this method, the particles of superabsorbent material that are present in remaining parts of the pulp mat remain only loosely bound in the structure. Neither is it possible with this method to obtain a structure in which only one layer contains superabsorbent material and where this layer is surrounded by one or more layers which are free from such material, which is desired in some product applications.